The present invention relates in general to ultrasonic inspection methods and, more particularly, to an ultrasonic inspection method for measuring the thickness of non-welded (non-metallurgically bonded) cladding on the inside surface of a vessel from an outside diameter surface of the vessel.
Cladding is applied to the interior walls of process vessels to protect the vessel base material, typically mild carbon steel, from the vessel contents. This cladding approach allows the vessels to be constructed without the expense of using the typically exotic cladding materials to make up the entire vessel wall through thickness. If the cladding fails or erodes/corrodes away, the vessel contents may quickly degrade the vessel base material. Accordingly, clad thickness is periodically examined typically by venting the vessel and cleaning it to allow human entry. The cladding inspection is typically performed from the interior surface, and is done visually or through some qualitative measure of clad thickness such as using electromagnetics.
U.S. Pat. No. 4,669,310 to Lester, assigned to The Babcock & Wilcox Company, is drawn to a method for ultrasonically measuring high temperature oxide scale on the inner surface of a fluid containing tube, and may be referred to as the NOTIS.RTM. inspection technique. NOTIS.RTM. is a registered service mark of The Babcock & Wilcox Company for "inspecting and testing boiler tubes used for steam generation". An ultrasonic pulse is directed into the tube and the time of flight of the ultrasonic pulse within the scale is determined. The time of flight is correlated to the thickness of the scale. First and second time of flights to and from the tube metal/scale interface and to and from the scale/fluid interface are determined. The difference between the first and second times of flight are correlated to determine the thickness of the scale. A high frequency transducer capable of frequencies of 50 MHz is employed, the transducer having a circular active element with a diameter of 0.250". A high frequency pulser/receiver is provided to produce high frequency pulses having a short duration with a wide (60 MHz) bandwidth. This allows production and receipt of an ultrasonic signal that is capable of resolving the energy reflected from both the tube/scale and scale/fluid interfaces. As indicated at col. 4, lines 57-69, Lester teaches that the ultrasonic test frequency employed depends upon the nominal value of the scale thickness to be measured. To resolve the interfaces, the scale thickness must be at least one wavelength of the ultrasound. Based upon a conservative approximation of the velocity of sound in steel, Table 2 of the '310 patent shows the various ultrasonic frequencies, in MHz, and the minimum scale thicknesses which can be resolved at these frequencies. The '310 patent states that the velocity of sound in scale is generally not known and will vary in scales of different compositions, which results in the fact that the time of flight technique does not produce an absolute or exact scale thickness. However, time of flight data can be related to actual scale thickness measurements established by physical techniques such as metallurgical examinations. Testing cited in the '310 patent indicates that frequencies on the order of 5-10 MHz could not be used to measure the thickness of oxide scale although testing did indicate that a highly damped 10 MHz transducer with a laboratory grade pulser/receiver and oscilloscope could detect but not measure the presence of scale on the inner surface of a boiler tube when the thickness of the scale was greater than 0.007". The scale being interrogated is produced through oxidation of the boiler tube material when exposed to high temperature, in contrast to a scale caused by accumulation of other materials on the inside of the tube, or an intentionally applied layer of material such as cladding.
Mitsubishi Japanese patent specification 60-142210 teaches a method for measuring the thickness of composite material made of zirconium alloy and zirconium using an ultrasonic wave and prescribed frequency in a line focus type probe. The tube 1 has an outer layer of material 1a made of zirconium alloy and an inner layer of material 1b made of zirconium. The tube 1 is dipped in a fluid medium 2 and an ultrasonic wave having a frequency from 20-100 MHz is emitted from the probe 3. Transmitted and reflected waves are displayed on a cathode ray tube 5 of an ultrasonic inspector 4. The ultrasonic wave is reflected at the interface between the outer layer 1a and the medium 2, as well as between the outer layer 1a and inner layer 1b, generating echoes. The thickness of the layers 1a and 1b are thus measured accurately and surely from the time intervals between the respective echoes. The Mitsubishi approach disclosed therein is thus an immersion technique, wherein the test pieces are immersed in a fluid medium for measuring the thickness of composite materials made of zirconium alloy and zirconium (i.e., a clad tube) using an ultrasonic wave and a prescribed frequency from 20-100 MHz. Ultrasonic waves are reflected at the interface between the outer and inner layers and the thicknesses of each of these layers are measured by the time interval between the echoes.
Takahashi et al. (U.S. Pat. No. 4,334,433) teaches a method and apparatus for measuring the thickness of clad steel which involves applying ultrasonic waves on the side of a sample opposite the cladding layer. Reflective waves are sensed and displayed on an oscilloscope and using a disclosed technique for eliminating noise and interference, the position of the interface between the cladding layer and the base metal layer are measured from a pulse produced due to the discontinuity of acoustic impedances at the interface. One disclosed embodiment is a case where the base metal is made of carbon steel having a small difference in its acoustic impedance from that of a cladding layer thereon which is made of stainless steel with accordingly only a very weak echo generated at the interface therebetween. Takahashi et al. teaches that it should be noted in the case of other cladding materials such as aluminum, copper, and alloys thereof, the differences of their acoustic impedances from that of the base metal are much greater than in this example and it is easier to measure the thicknesses in such cases. According to the invention, a weak interface echo is clearly separated just in front of the bottom surface echo with the precision of at least ten times that previously obtainable by amplifying the gain, enhancing the pulse output, and displaying the shape of the echo signal on an oscilloscope tube. Measurement of the thickness of any position of the clad steel within plus or minus 1 mm is obtained, from either the surface side or the back side. In particular, if the total thickness of the clad steel is more than 2.5 mm and that of cladding material is more than 0.4 mm, it is stated to be quite easy to measure both thicknesses. It is also stated to be possible to measure thickness of a material having a curvature of more than 1.5 times that of the contact detecting terminal diameter which has a cylindrical or arcuate shape. Takahashi et al thus teaches a method and apparatus for measuring the thickness of clad steel which involves applying ultrasonic waves on the side of the sample opposite the cladding layer. The technique is particularly applicable to cases wherein the base metal is made of carbon steel having a small difference in its acoustic impedance from that of a cladding layer thereon made of stainless steel. In such a case, a very weak echo is generated at the interface therebetween. Takahashi et al can measure the thickness of the clad steel within plus or minus 1 mm.
Desruelles et al. (U.S. Pat. No. 4,918,989) teaches a method for checking the thickness of the plating on a metal tube where the plating to be checked is at least 0.4 mm thick and in which the acoustic impedance differs by at least 1% relative from that of the core of the tube. A properly dampened transducer is selected which has a frequency of 4-10 MHz. To determine the thickness of the plating, at least one double echo from the interface between the plating and the tube cores used, or alternatively a triple echo from the interface is used. Properly dampened transducers are selected which have a frequency of 4-10 MHz. Col. 2 of this patent indicates that its principal frequency is between 4 and 20 MHz. Desruelles et al. requires the tube to be pushed through an immersion tank.
Heumuller (U.S. Pat. No. 4,603,583) discloses a method for the ultrasonic testing of ferritic parts having a cladding. This patent is primarily drawn to flaw detection wherein the faults are located near the base metal and cladding interface, and involves disposing an ultrasound transmitter on a side of the part opposite the cladding which radiates longitudinal waves therefrom into the body at an angle between 70.degree. and 86.degree. relative to the perpendicular. The patent states that the longitudinal waves release a wave at the cladding plane traveling parallel to the interface which has never before been detected and is only minimally sensitive to structure-related backscatter so that a correspondingly large margin or ratio of signal to noise is obtained. The radiated angle can be adjusted to focus the ultrasound beam on the cladding to enhance sensitivity.
Desruelles (U.S. Pat. Nos. 5,329,561 and 5,225,148) discloses a device for checking the thickness and the cohesion of the interface of a duplex tube comprising a tubular core made from an alloy such as a zirconium alloy and covered with a covering or cladding layer made from an alloy the base metal of which is identical to the base metal of the alloy constituting the tubular core. The device is used to check geometrical dimensions of the duplex tube and, in particular, its total thickness, the thickness of the covering and cladding layer, and to detect flaws and cohesion at the interface between the covering or cladding layer and the tubular core. The method of the '561 patent combines ultrasonic and magnetic induction steps to measure the thickness of the covering layer. The total thickness of the tube is calculated from the measurements of the propagation times of the ultrasonic waves and of the thickness of the covering layer, and the cohesion of the tube at its interface is determined by analyzing the amplitude and the shape of the ultrasonic waves reflected by the covering or cladding layer. The Desruelles '561 patent, in the Background of the Invention section at col. 2, discloses that there are known ultrasonic wave "pulse-echo" techniques to check the thickness of a duplex tube based on a zirconium alloy. In one such technique, limitations are identified that measuring of cladding thicknesses of less than 0.4 mm are not possible because the ultrasonic waves used have a frequency which does not exceed 20 MHz. For cladding layers whose thickness lies between 80 and 100 .mu.m, it is stated that ultrasonic waves at a very high frequency in the order of 100 MHz would be required. Further, in the case of jackets for fuel rods, the cladding layer and the tubular core of the duplex tubes have very similar acoustic properties, and the coefficient of reflection of the acoustic waves at the cladding/core interface is very small (generally less than 2%). The interface echo is then very small and becomes drowned out in the acoustic and electronic noise of the ultrasonic signal.
Jackson (U.S. Pat. No. 4,446,736) discloses an ultrasonic testing method to determine the integrity of the internal lining of a hollow body, especially a pipeline, by transmitting an ultrasonic pulse from the exterior of the body through the adjacent wall of the body and fluid medium therein and monitoring the wave, if any, reflected from the opposite wall. The technique is particularly adapted to determine the integrity of concrete linings within a pipe. An absence of a reflected wave indicates that the lining is intact or partially intact, while a reflected wave frequently indicates loss of the lining on an adjacent wall. However, the patent acknowledges that sometimes a reflection from the opposite wall may occur, even when the lining is intact, and in such cases comparison of the reflected wave with predetermined standards can provide an indication of whether or not the lining is indeed intact. This latter situation would involve comparing the magnitude of the reflected wave with a predetermined standard to assess the presence or absence of the lining of the wall at the point where the transmitted wave enters the wall.
U.S. Pat. No. 5,349,860 to Nakano, deceased et al. discloses an apparatus for measuring the thickness of a clad material having an outer mother metal and inner clad metal. The apparatus includes a transmitter crystal and a receiver crystal of a double crystal angle-type probe which contacts the outer surface of the mother metal, for receiving a first echo from the boundary surface of the mother metal and clad metal, and for receiving a second echo from the inner, bottom surface of the clad metal. The echo signals are amplified, a detector detects the zero-crossing points of the echoes, and the thicknesses of the clad material and clad metal are based upon the calculated periods of time for the zero point crossings. The frequency range cited is preferably in the range from 2-10 MHz, and coupling methods can be used including the contact method and the water column coupling method. Stationary, spiral or longitudinal scans can be performed.
However, Nakano, deceased et al., does not consider three problems that will be encountered in the actual application of the technique described in that patent. First, the approach disclosed depends on the setting of the "C-gate" in the region of time where the reflection, C', from the clad will be present. A user of this approach thus must have prior knowledge of the general expected time of arrival of the C' signal (which implies that the user must know the approximate clad thickness before the thickness measurement can be made) in order to define the setting of the C-gate.
Second, a user of the Nakano, deceased et al., approach assumes that the only signal present in the "C-gate" is the reflected signal from the clad interface. As illustrated in FIG. 5 of U.S. Pat. No. 5,349,860, this is an idealized depiction of the assumed reflected signals. In practical applications, there will be many reflected signals within the "C-gate" time span due to the presence of reflections from possible grain boundaries in the material and from electronic noise interference.
Third, the idealized approach of the '860 patent does not address practical applications, especially with higher frequency ultrasonic sensors or ultrasonic sensors with delay lines. Such ultrasonic sensors, especially those operating at high (15-100 MHz) frequencies, typically have artifact signals which are present even when the ultrasonic sensor is not applied to a test piece, due to small imperfections which occur during manufacture of the ultrasonic sensors. For applications requiring delay lines to provide temperature isolation, there are additional reflected signals present which arise from the sensor/delay line interface that can occur in the general time frame or area as the expected clad reflected signal. The '860 patent does not provide any method for discriminating among these various signals.
The above-identified patents typically deal with metallurgically-bonded materials. If the cladding is metallurgically connected, the laws of physics define the ratio of incident to reflected wave strength (i.e., reflectivity). FIG. 1 represents a bar chart of the values of the calculated ratios of incident to reflected wave strength (i.e., reflectivity) for different cladding materials on a mild steel base material. As will be noted from FIG. 1, for many cases the reflectivity is very small. For those approaches relying on differences of impedances, there are some base metal/cladding combinations that would be difficult if not impossible to ultrasonically inspect.
Some of the aforementioned patents cited above involve immersion-type ultrasonic techniques. Such techniques are not readily applicable for ultrasonic cladding thickness measurements on large (20-30 feet diameter, 100-150 feet tall) structures, particularly those that remain in operation. Further, traditional industry practices do not use contact methods at very high frequencies.
Ultrasonic methods for detecting the thickness of non-welded cladding on the inside surface of process vessels from the outside has been attempted by personnel in the chemical industry, but the clad/base material interface apparently could not be uniquely identified because of limitations in the approach. Referring to FIG. 2, in this approach a commercial ultrasonic test instrument 10 having a bandwidth of 20 MHz and a 20 MHz transducer 12 was used to inspect a workpiece 14 having a base metal layer 16 and an inner, non-welded cladding layer 18. On the oscilloscope display (FIG. 3) with this system, large amplitude signals are seen representative of the initial ultrasonic pulse "main bang" 20 and of the reflection 22 from the cladding/air interface at the inside wall surface of the process vessel. However, the amplitude of the signal representative of the reflection at the base metal/cladding interface (which would occur before (i.e., earlier in time and thus to the left of the displayed cladding/air interface signal) could not be distinguished and seen above the noise floor. It is believed that the rectified signals obtained with such a system do not allow the small amplitude signal representative of the base metal/cladding interface to be picked up visually.
It is thus apparent that a need still exists for an accurate, reliable ultrasonic inspection method for measuring the thickness of non-welded cladding on the inside surface of a process vessel from an outside diameter surface of the vessel.